How to change the color of clipping border display?
There is a setting for 'Clipping Border' now in newer version of Blender.
themes / preferences / 3dview / clipping border color option box.
The default color is a dark grey, and the setting for transparency is called 'A' for Alpha Value. The default Alpha is 1 which is Opaque, if you move the slider to '0' the View Clipping Background is Transparent!